prevent overriding aws elastic handler when set

[SylvainJuge]

What does this PR do?

Fixes #2969 by not overriding the ELASTIC_APM_AWS_LAMBDA_HANDLER environment variable.

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.asciidoc
  • I have added tests that prove my fix is effective or that my feature works
  • Added an API method or config option? Document in which version this will be introduced
  • I have made corresponding changes to the documentation

[apmmachine]

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Build stats

• Start Time: 2023-06-26T14:07:04.101+0000

• Duration: 15 min 3 sec

❕ Flaky test report

No test was executed to be analysed.

🤖 GitHub comments

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[SylvainJuge]

@AlexanderWert is that what you meant in the related issue comment ?

[AlexanderWert]

@AlexanderWert is that what you meant in the related issue comment ?

yes, exactly

[github-actions]

/test